Method for producing a thin-film solar cell by use of microcrystalline silicon and a layer sequence

ABSTRACT

A thin film solar cell is disclosed comprising the following layers deposited on a substrate:
         a microcrystalline p- or n-layer,   an intermediate microcrystalline silicon i-layer applied by a hot-wire chemical-vapor deposition (HWCVD) method on the microcrystalline p- or n-layer a),   an additional i-layer of microcrystalline silicon, which is formed by depositing on the intermediate microcrystalline silicon i-layer, by a plasma enhanced chemical vapor deposition (PECVD), a sputtering process, or a photo-CVD method whereby layers b) and c) together form an i-layer, and   if a p-layer is present as the layer of step a), an n-layer, and if an n-layer is present as the layer of step a), a p-layer that is either microcrystalline or amorphous.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/793,690 filed 21 Jun. 2007, now allowed, which is the US NationalPhase of PCT Application PCT/DE2005/002237 filed 13 Dec. 2005 with aclaim to the priority of the German National Phase Application DE 102004 061 360.5 filed 21 Dec. 2004.

FIELD OF THE INVENTION

The invention relates to a method for producing a thin-film solar cellby use of microcrystalline silicon and a layer sequence.

BACKGROUND OF THE INVENTION

Microcrystalline thin-film solar cells comprise various layers ofsilicon that differ in their doping and degree of crystallinity. Thelayers are deposited on substrates such as glass. Various methods areknown for this purpose. In the PECVD method a gas mixture is decomposedinto fragments in a plasma, and in the HWCVD method the gas or gasmixture is decomposed by heating a wire. In both methods decompositionproducts deposit on the substrate, resulting in a coating. The photo-CVDmethod and sputtering, known from the prior art, may also be used todeposit layers on surfaces.

A distinction is made between pin cells and nip cells, that differ intheir layer sequence.

In a pin solar cell the layer adjoining the transparent substrate is ap-layer, i.e. a p-doped silicon layer; the i-layer is located on thep-layer and is not doped. The n-layer, that is n-doped, is located onthe i-layer. In a nip solar cell the p-layer and the n-layer areexchanged with one another, and likewise include an i-layer.

The microcrystalline p-layer is p-doped silicon that is doped withboron, for example.

One skilled in the art is familiar with the manner in which these layersare deposited.

For a p-layer, gas mixtures containing silicon compounds, positivelydoped compounds such as boron compounds, as well as hydrogen gas areused that are decomposed and deposited on the surface. The resultinglayer is microcrystalline. The known HWCVD, PECVD, and photo-CVD methodsmay be used for this purpose.

For the deposition of the microcrystalline i-layer, a mixture comprisinga silicon compound and hydrogen is decomposed, whereupon amicrocrystalline Si layer is deposited. The known HWCVD, PECVD,sputtering, and photo-CVD methods may likewise be used for this purpose.

For the deposition of the microcrystalline n-layer, a mixture comprisinga silicon-containing compound, hydrogen, and negatively doped compoundssuch as phosphorus-containing compounds is decomposed. The HWCVD, PECVD,and photo-CVD methods may be used here as well.

According to one known method, thin-film solar cells are deposited ontotransparent substrates by means of PECVD. The deposition sequence is pinor also nip, the individual layers generally being deposited indifferent cavities. For PECVD the microcrystalline layers are depositedby use of various possible excitation frequencies, such as radiofrequencies, microwave, or very high frequency (VHF). High plasmaoutputs are necessary for achieving high deposition rates by means ofPECVD. The HWCVD method requires high wire temperatures for achievinghigh deposition rates. Solar cells having a low fill factor and a lowopen circuit voltage have heretofore been produced at high depositionrates. The HWCVD method is also much more time-intensive than the PECVDmethod. Solar cells having very high Voc and FF may be obtained at lowsubstrate temperatures.

OBJECT OF THE INVENTION

The object of the invention is to provide a method for producing solarcells that results in solar cells having higher efficiency, inparticular higher Voc and FF. An aim is to increase the open circuitvoltage of the solar cells by use of the method. A further aim is toprovide a layer sequence having characteristics that allow higher opencircuit voltages and fill factors. It should be possible to carry outthe method in the shortest possible time.

SUMMARY OF THE INVENTION

The method according to the invention may be used to produce pin and nipsolar cells based on microcrystalline silicon that have a higherefficiency, i.e. a higher open circuit voltage (Voc) and a higher fillfactor (FF).

Advantageous refinements of the invention are described in thesubclaims.

The invention is explained below.

For the production of the solar cells or layer sequences according tothe invention, in a first step a transparent current-conducting layer isdeposited on a transparent substrate, for example glass or plastic,which is referred to in the technical field as a TCO layer. This layertypically has a layer thickness of 0.5-2 μm, and may have, but is notlimited to, the following composition: SnO₂, ZnO, ITO.

The deposition may be carried out using conventional methods, forexample sputtering or CVD.

The method according to the invention is explained below for a pin solarcell. However, the invention also encompasses the production of a nipsolar cell. In the production of the latter the same steps are carriedout, except that the sequence of application of the p-layer and then-layer is reversed.

In a further step, on the TCO layer a microcrystalline p-layer isdeposited onto the transparent substrate together with the superposedTCO layer, preferably by use of the PECVD method. However, thedeposition may also be performed using other methods known from theprior art, such as the HWCVD method, sputtering, or the photo-CVDmethod.

This step is carried out using method parameters and gas mixtures knownto one skilled in the art.

Gas mixture components comprise a silicon-containing compound, hydrogen,and a compound that deposits a substance that results in p-doping.

The silicon-containing compound may be, but is not limited to, at leastone component from the group comprising SiH₄, methyl silane, dimethylsilane, trimethyl silane, tetramethyl silane, disilane, trisilane, orhalosilanes such as HSiCl₃, H₂SiCl₂, H₃SiCl₁, SiCl₄, or thecorresponding fluorine compounds.

The concentration of the silicon compounds is, for example, between 0.1%and 10%.

The total pressure may be, for example, between 0.2 hPa and 20 hPa.

The compound that deposits the component that results in p-doping maybe, but is not limited to, at least one component of the groupcomprising diborane, trimethylborane, or organyl compounds of Ga and Al,such as trimethyls or triethyls.

The concentration of the p-doped compound is, for example, between 0.1%and 2%, relative to the silicon-containing compound.

The gas mixture, which comprises the silicon-containing compound, thecompound that deposits the component that results in p-doping, andhydrogen as substitutes is decomposed by means of the PECVD method oranother method such as the HWCVD method or the photo-CVD method, and theresulting components deposit in the form of a microcrystalline p-dopedsilicon layer.

The substrate temperature may, for example, be between 50° C. and 300°C.

The depositing layers have a layer thickness of 2 nm-100 nm, preferably5 nm-30 nm, particularly preferably 10 nm-20 nm.

In the next step, according to the invention a microcrystalline siliconlayer is deposited by the HWCVD method. Surprisingly, it has been shownthat the deposition of a layer of microcrystalline silicon by the HWCVDmethod results in an increase in the open circuit voltage and the fillfactor, and thus the efficiency of solar cells. The layer thickness ofthe layer deposited by means of the HWCVD method may be 2 nm-200 nm,preferably 5 nm-50 nm, particularly preferably 10 nm-20 nm.

For the deposition of the above-referenced layer, a gas comprisinghydrogen and a silicon-containing compound that may be, but is notlimited to, SiH₄, disilane, trisilane, or halosilanes such as HSiCl₃,H₂SiCl₂, H₃SiCl₁, SiCl₄, or the corresponding fluorine compounds isused.

For carrying out the method step according to the invention, theparameters preferably are, but are not limited to, the following:

Wire material: tungsten, tantalum, graphite, rhenium, or osmium, orhigh-melting materials.

Wire temperature: 1200° C.-2200° C. (or preferably 1500°-1800° C.)

Substrate temperature: <400° C. (<300° C. in pin)

Pressure: 1 Pa-100 Pa (or preferably 2 Pa-10 Pa)

Hydrogen dilution: 0.1%-20% silane in hydrogen.

The layer produced according to these parameters (produced by HWCVD) hasa thickness of 2 nm-200 nm, preferably 5 nm-20 nm, and has acrystallinity of >0%.

The substrate temperatures are preferably between 50° C. and 300° C.,particularly preferably between 150° C. and 200° C.

The gas concentrations are, for example, 0.1-10% silane orsilicon-containing compound, relative to hydrogen.

The total pressure range is between 1 Pa and 100 Pa.

This microcrystalline silicon layer applied as an intermediate layercauses the overall solar cell to have a higher open circuit voltage anda higher, fill factor, and thus a higher efficiency. This layer forms apart of the microcrystalline i-layer that is applied to the p-dopedlayer.

The absolute efficiency is increased by up to 0.8%, the open circuitvoltage by up to 25 mV, and the fill factor by up to 3%.

In a further step, preferably by use of the PECVD method, an additionalmicrocrystalline silicon i-layer is deposited. In this method step, thesame as for the other steps, alternative methods such as HWCVD,sputtering, or photo-CVD may be used.

In principle, the deposition may be carried out using the samecomponents as for the deposition of the intermediate layer.

The compounds used for the deposition are hydrogen and at least onecomponent from the group comprising, but not limited to, SiH₄, disilane,trisilane, or halosilanes such as HSiCl₃, H₂SiCl₂, H₃SiCl₁, SiCl₄, orthe corresponding fluorine compounds.

This layer preferably has a crystallinity of >20%.

The silane concentrations or the concentration of the silicon-containingcompound may be, for example, 0.1%-100%. Hydrogen may optionally bepresent.

The temperature range is preferably 50-400° C., particularly preferably150° C.-200° C. for pin layer sequences, or 150°-250° C. for nip layersequences.

The total pressure is, for example, between 0.2 hPa and 20 hPa.

The layer thickness of the microcrystalline i-layer deposited by thePECVD method is preferably between 0.2 μm and 10 μm, particularlypreferably between 0.5 μm and 2 μm.

Characteristics of the i-layer:

Produced by PECVD (RF, VHF, microwave, etc.)

Thickness: preferably 0.5 μm-5 μm

Crystallinity: >20%.

The layer deposited by use of the HWCVD method and the PECVD methodforms an i-layer of microcrystalline silicon, which compared to ani-layer according to the prior art allows a higher open circuit voltage,a higher fill factor, and a higher efficiency.

An n-layer is deposited on the i-layer.

The n-layer may be amorphous or microcrystalline.

To this end, a gas mixture comprising a silicon-depositing compound,optionally hydrogen, and a component that causes n-doping is used.

The component used as the silicon-depositing component may for examplebe, but is not limited to, SiH₄, methyl silane, dimethyl silane,trimethyl silane, tetramethyl silane, or HSiCl₃, H₂SiCl₂, H₃SiCl₁, orSiCl₄.

Phosphine, for example, may be used as an n-doping compound.

The deposition of the n-doped layer may be carried out using the PECVDmethod, photo-CVD method, sputtering, or HWCVD method, for example.

The layer thickness of the n-layer is preferably between 5 nm and 50 nm,particularly preferably between 10 nm and 30 nm.

The preferred coating temperatures are between 100° C. and 400° C.

The total pressure is typically between 0.2 hPa and 20 hPa.

The concentration of the silicon-containing compound in the gas phase isbetween 0.1% and 100%.

Phosphine is used in a concentration range that is preferably between0.1% and 5%, relative to silane or the silicon-containing compound.

The percentage values in each case refer to volume percent.

The topmost layer, i.e. for pin the n-layer and for nip the p-layer, isusually coated with a reflective layer. This reflective layer may, forexample, be composed of a TCO layer and a metal layer made of Ag or Al,for example.

The invention also encompasses stacks of layers that contain the layersequence according to the invention at least once.

The invention also encompasses layer sequences having the followingpatterns:

A) a microcrystalline p-layer,

-   -   a microcrystalline intrinsic silicon layer applied by the HWCVD        method,    -   an i-layer applied by the PECVD method, and    -   a microcrystalline or amorphous n-layer        or also a thin-film solar cell comprising:    -   a transparent substrate,    -   a TCO layer,    -   a microcrystalline p-layer,    -   a microcrystalline intrinsic silicon layer applied by the HWCVD        method,    -   an i-layer applied by the PECVD method,    -   a microcrystalline or amorphous n-layer, and    -   a reflective layer.

B) a microcrystalline n-layer.

-   -   a microcrystalline intrinsic silicon layer applied by the HWCVD        method,    -   an i-layer applied by the PECVD method, and    -   a microcrystalline or amorphous p-layer        or also a thin-film solar cell comprising:    -   a transparent substrate,    -   a TCO layer,    -   a microcrystalline n-layer,    -   a microcrystalline intrinsic silicon layer applied by the HWCVD        method,    -   an i-layer applied by the PECVD method,    -   a microcrystalline or amorphous p-layer, and    -   a reflective layer.

The p, n, and TCO layers are conventional, and are not limited to theparameters and production methods illustrated in the description.

EXAMPLE

A 5-50 nm thick intrinsic HW layer was deposited on the p-layer that hadbeen deposited onto a ZnO substrate by VHF-PECVD. This layer wasdeposited at a wire temperature of 1650° C., 3.5 Pa, and 2-10% silane inhydrogen, at a deposition rate of 1-2 Å/s. The production of the solarcell was continued with the deposition of the i-layer by VHF-PECVD at ahigh growth rate. The open circuit voltage was thus increased by 20 mV,the fill factor by 2%, and the absolute efficiency by 0.8%.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantageous characteristics of the solar cell according to theinvention are shown in the figures:

FIG. 1 illustrates the comparison of the open circuit voltages of pinsolar cells with and without the intermediate layer according to theinvention.

The silane concentration for the i-layer deposition is plotted on theabscissa in %.

The ordinate shows the open circuit voltages obtained, in mV.

The circles represent solar cells without the intermediate layeraccording to the invention, and the triangles represent solar cells withthe intermediate layer according to the invention. It is seen that theopen circuit voltage is increased according to the invention.

FIG. 2 shows the comparison of the fill factors of pin solar cells withand without the intermediate layer according to the invention.

The silane concentration for the i-layer deposition is plotted on theabscissa in %.

The ordinate shows the fill factors obtained, in %.

The circles represent solar cells without the intermediate layeraccording to the invention, and the triangles represent solar cells withthe intermediate layer according to the invention. It is seen that thefill factors are increased according to the invention.

The invention claimed is:
 1. A thin film solar cell comprising thefollowing layers deposited on a substrate: a) a microcrystalline p- orn-layer, b) an intermediate microcrystalline silicon i-layer having athickness of 10 nm to 20 nm and a crystallinity of >0% and applied by ahot-wire chemical-vapor deposition (HWCVD) method on themicrocrystalline p- or n-layer a), c) an additional i-layer ofmicrocrystalline silicon, having a thickness of 0.5 μm to 2 μm and acrystallinity of >20% and which is formed by depositing on theintermediate microcrystalline silicon i-layer, by a plasma enhancedchemical vapor deposition (PECVD), a sputtering process, or a photo-CVDmethod whereby layers b) and c) together form an i-layer, and d) if ap-layer is present as the layer of step a), an n-layer, and if ann-layer is present as the layer of step a), a p-layer that is eithermicrocrystalline or amorphous.
 2. The thin film solar cell according toclaim 1 wherein the microcrystalline p-layer is situated on a TCO layerthat is applied to a transparent medium.
 3. The thin film solar cellaccording to claim 1, further comprising a reflective layer or a stackof additional layers above the n-layer.
 4. The thin film solar cellaccording to claim 1 wherein the microcrystalline n-layer is on a TCOlayer that is applied to a transparent medium.
 5. The thin film solarcell according to claim 1, further comprising a reflective layer or astack of additional layers above the p-layer.